High-strength aluminum alloy sheet

ABSTRACT

An Al—Mg—Si aluminum alloy sheet has a chemical composition which includes at least one element selected from the group consisting of Mn, Cr, and Zr and which meets the condition: 0.6≤([Mg]/[Si]-0.15[Mn]-0.3[Cr]-0.1[Zr]≤1.8. The aluminum alloy sheet is controlled to have a higher aspect ratio of grains, thereby has significantly higher strength after artificial aging such as paint bake (after bake hardening), and satisfactorily has strength necessary for automobile structural components, without deterioration in bendability.

TECHNICAL FIELD

The present invention relates to Al—Mg—Si aluminum alloy sheets. As used herein, the term “aluminum alloy sheet” refers to a rolled sheet such as a hot-rolled sheet or a cold-rolled sheet, after subjected to heat treatments such as solution treatment and quenching (temper; T4), but before forming into a structural component to be used. Hereinafter, “aluminum” is also simply referred to as “Al”.

BACKGROUND ART

Social demands for weight reduction of vehicles such as automobiles have become higher and higher in consideration typically of global environment. To meet these demands, aluminum alloy materials are increasingly applied as automobile materials instead of ferrous materials such as steel sheets, because the aluminum alloy materials have formability and bake hardening properties (bake hardenability) at excellent levels and have lighter weights. The “bake hardening” is hereinafter also referred to as “BH”.

Representative examples of aluminum alloy sheets for automobile large-sized panel components such as outer panels and inner panels include AA or JIS 6xxx-series aluminum alloy sheets, which are Al—Mg—Si alloy sheets. The AA or JIS 6xxx-series are hereinafter also simply referred to as “6xxx-series”. The 6xxx-series aluminum alloy sheets have chemical compositions essentially containing Si and Mg and have low yield strength (low strength) upon forming (shaping) to surely offer good formability. In addition, the 6xxx-series aluminum alloy sheets have excellent bake hardenability, in which they have higher yield strength (strength) by heating in artificial aging (hardening) treatments such as paint bake of panels, which are performed after forming, to surely offer required strength.

For further weight reduction of automobile bodies, demands are made to apply aluminum alloy materials further to other automobile components than the panel components, where the other automobile components are exemplified by automobile structural components including frame components such as frames and pillars; and reinforcements such as bumper reinforcements and door beams.

However, these automobile structural components require still higher strength as compared with the automobile panels. To be applied to the frame components or reinforcements, the 6xxx-series aluminum alloy sheets, which have been applied to the automotive body panels, require still higher strength.

However, it is not so easy to achieve such higher strength of conventional 6xxx-series aluminum alloy sheets without significantly changing chemical compositions and production conditions and without deterioration in properties such as bendability.

Various techniques on control of grain aspect ratios of 6xxx-series aluminum alloy sheets have been proposed as control of sheet microstructures, so as to improve properties, such as bake hardenability, necessary for the panel components.

For example, Patent Literature (PTL) 1 proposes a technique to offer better formability into the automotive body panels. According to this technique, a 6xxx-series aluminum alloy sheet after heat treatments including solution treatment and quenching is controlled to have an average size of grains of 60 μm or less, where the grains are defined by large angle grain boundaries having a misorientation of a tilt angle of 15° or greater. In addition, this 6xxx-series aluminum alloy sheet is controlled to have a standard deviation in grain size distribution of 15 pun or less and to have a low aspect ratio of 2.5 or less.

PTL 2 proposes a technique to allow a high-strength 5xxx-series aluminum alloy sheet to have better formability into the automotive body panels. According to this technique, the 5xxx-series aluminum alloy sheet is controlled to have a high aspect ratio of grains of 3.0 or more to thereby have better strength-elongation balance.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2009-173973

PTL 2: JP-A No. 2009-24219

SUMMARY OF INVENTION Technical Problem

The aluminum alloy sheets according to the conventional techniques to control aspect ratios are intended to offer better formability into the target panel components, but have inferior strength when to be used as the frame components or reinforcements.

In contrast, attempts may be made to perform the artificial aging at a higher temperature so as to offer higher strength. However, there are limitations and restrictions on the temperature of the artificial aging (hardening) treatment due typically to treatment efficiency and cost, deterioration of the baked paint, and reduction in strength due to over-aging; and there are circumstances to fail to achieve such higher temperature. Thus, aluminum alloy sheets have a ceiling on strength after BH.

The frame components or reinforcements do not require such high press formability as with the panel components, but require formability in forming the material sheets into the automobile structural components, where the formability is exemplified by such bendability as not to be broken by V bending. The material aluminum alloy sheets therefore require higher strength without deterioration in the formability.

The present invention has been made to solve these problems and has an object to provide a 6xxx-series aluminum alloy sheet which can be used for, and can be formed into, automobile structural components such as the frame components or reinforcements, which can have higher strength without significantly changing chemical compositions and production conditions of conventional 6xxx-series aluminum alloy sheets.

Solution to Problem

To achieve the object, the present invention provides a high-strength aluminum alloy sheet as follows. The aluminum alloy sheet is an Al—Mg—Si aluminum alloy sheet containing, in mass percent, Mg in a content of 0.6% to 2.0%, Si in a content of 0.6% to 2.0%, and at least one element selected from the group consisting of Mn, Cr, and Zr in a total content of 0.2% to 1.0%, where the contents of Mg, Si, Mn, Cr, and Zr meet the condition represented by the balance formula: 0.6≤([Mg]/[Si]-0.15[Mn]-0.3[Cr]-0.1[Zr])≤1.8, where [Mg], [Si], [Mn], [Cr], and [Zr] are the contents (in mass percent) respectively of Mg, Si, Mn, Cr, and Zr, with the remainder consisting of Al and unavoidable impurities. The aluminum alloy sheet has a D1 of 100 μm or less and an aspect ratio D1/D2 of 3.0 to 10.0, where D1 is the average length of major axes of grains in an area of 1000 μm by 1000 μm as observed by SEM-EBSD in a cross section parallel to the sheet rolling direction; and D2 is the average length of minor axes of the grains.

Advantageous Effects of Invention

In the present invention, the relationship between grain aspect ratio and bake hardenability of a 6xxx-series aluminum alloy sheet was reconsidered, on the precondition that the aluminum alloy chemical compositions and production conditions of conventional aluminum alloy sheets are not significantly changed. As a result, it was found that the 6xxx-series aluminum alloy sheet has better bake hardenability with an increasing grain aspect ratio; and that the resulting aluminum alloy sheet can have a higher 0.2% yield strength of 300 MPa or more without deterioration in bendability, even after natural aging at room temperature, where the 0.2% yield strength is determined alter 2% stretching and subsequent BH treatment at 185° C. for 20 minutes.

Control of the grain aspect ratio of such 6xxx-series aluminum alloy sheets is publicly known as described above, but the relationship between the grain aspect ratio and bake hardenability has not yet been known well.

With the present invention, a 6xxx-series aluminum alloy sheet can have significantly higher strength after artificial aging such as paint bake (alter BH) without deterioration in bendability, by a simple technique of controlling the 6xxx-series aluminum alloy sheet to have a higher grain aspect ratio. This configuration allows the 6xxx-series aluminum alloy sheet to satisfactorily have strength necessary for automobile structural components, which strength is higher as compared with the strength necessary for automotive body panels.

DESCRIPTION OF EMBODIMENTS

Some embodiments of the present invention will be specifically illustrated about individual required conditions below.

Chemical Composition

Initially, the chemical composition of the Al—Mg—Si aluminum alloy sheet according to the present invention will be illustrated below. Hereinafter the Al—Mg—Si aluminum alloy sheet is also referred to as a “6xxx-series aluminum alloy sheet”. With the present invention, the 6xxx-series aluminum alloy sheet is allowed to have a higher 0.2% yield strength of 300 MPa or more without significantly changing conventional chemical compositions and production conditions and without deterioration in bendability, where the 0.2% yield strength is determined after 2% stretching and subsequent BH treatment at 185° C. for 20 minutes, and where the resulting 6xxx-series aluminum alloy sheet may be used not for the panel components, but for the frame components or reinforcements.

To meet the condition in terms of chemical composition, the 6xxx-series aluminum alloy sheet has a chemical composition including, in mass percent, Mg in a content of 0.6% to 2.0%, Si in a content of 0.6% to 2.0%, and at least one element selected from the group consisting of Mn, Cr, and Zr in a total content of 0.2% to 1.0%, where the contents of Mg, Si, Mn, Cr, and Zr meet the condition represented by the balance formula: 0.6≤([Mg]/[Si]0.15[Mn]-0.3[Cr]-0.1[Zr])≤1.8, where [Mg], [Si], [Mn], [Cr], and [Zr] are the contents (in mass percent) respectively of Mg, Si, Mn, Cr, and Zr, with the remainder consisting of Al and unavoidable impurities.

The 6xxx-series aluminum alloy sheet may further include, in addition to the elements, at least one element selected from the group consisting of Cu in a content of 0.05% to 0.5%, Ag in a content of 0.01% to 0.2%, and Sn in a content of 0.005% to 0.3%. All percentages as contents of elements are in mass percent.

The content ranges and their significance, or permissible levels of the elements in the 6xxx-series aluminum alloy will be described below.

Si: 0.6% to 2.0% Silicon (Si) forms aged precipitates with magnesium (Mg) upon artificial aging such as paint bake treatment and allows the sheet to offer artificial aging hardenability, where the aged precipitates contribute to higher strength. This element is therefore essential for the strength (yield strength) necessary for the frame components or reinforcements. In addition, the element forms compounds typically with Mg and/or Mn, or with impurity Fe and effectively allows the aluminum alloy sheet to have a higher grain aspect ratio.

Si, if present in an excessively low content, forms smaller amounts of the aged precipitates as a result of artificial aging, and causes the aluminum alloy sheet to suffer from a smaller increase in strength during paint-bake cycles. In addition, Si in this case forms smaller amounts of the compounds during soaking and hot rolling and tends to cause grains formed upon recrystallization in solution treatment to be spherical, equiaxed grains having a grain aspect ratio of 3.0 or less. This causes the aluminum alloy sheet to have inferior strength.

In contrast, Si, if present in an excessively high content, forms coarse crystals typically with impurity Fe and causes the aluminum alloy sheet to have significantly inferior formability such as bendability. In addition, Si in this case causes the aluminum alloy sheet to be susceptible to hot rolling cracking in sheet production.

For these reasons, the Si content is controlled within the range of 0.6% to 2.0%.

Mg: 0.6% to 2.0%

Magnesium (Mg) also forms aged precipitates with Si, offers artificial aging hardenability, and is essential for high strength (yield strength), where the aged precipitates contribute to higher strength. In addition, this element forms compounds typically with Si and/or Mn, or with impurity Fe upon soaking and hot rolling and thereby effectively allows the aluminum alloy sheet to have a higher grain aspect ratio.

Mg, if present in an excessively low content, forms smaller amounts of precipitates after artificial aging and causes the aluminum alloy sheet to have inferior strength. In addition, Mg in this case forms smaller amounts of the compounds during soaking and hot rolling and tends to cause grains to be spherical, equiaxed grains having a grain aspect ratio of 3.0 or less upon recrystallization in solution treatment. This also causes the aluminum alloy sheet to have inferior strength.

In contrast, Mg, if present in an excessively high content, forms coarse crystals and coarse precipitates and causes the aluminum alloy sheet to have significantly inferior bendability.

For these reasons, the Mg content is controlled within the range of 0.6% to 2.0%.

At least one of Mn, Cr, and Zr: 0.2% to 1.0% in total.

Manganese (Mn), chromium (Cr), and zirconium (Zr) form compounds typically with Mg, Si, and/or Fe and effectively allow the aluminum alloy sheet to have a higher grain aspect ratio.

These elements, if present in an excessively low total content, form smaller amounts of the compounds during soaking and hot rolling and tend to cause recrystallization during solution treatment. This tends to cause grains to be spherical, equiaxed grains having an aspect ratio of less than 3.0.

In contrast, these elements, if present in an excessively high total content, form coarse compounds and cause the aluminum alloy sheet to have significantly inferior bendability.

For these reasons, at least one of Mn, Cr, and Zr is contained in a total content within the range of 0.2% to 1.0%.

Balance Formula

According to the present invention, the chemical composition is further controlled so that the contents of Mg, Si, Mn, Cr, and Zr meet the condition represented by the balance formula: 0.6≤([Mg]/[Si]-0.15[Mn]-0.3[Cr]-0.11[Ze])≤1.8, where [Mg], [Si], [Mn], [Cr], and [Zr] are the contents (in mass percent) respectively of Mg, Si, Mn, Cr, and Zr.

As described above, Mg and Si form aged precipitates, which contribute to strengthening (hardening). It is therefore desirable to control the balance between solute Mg and solute Si within a specific range, in order to form aged precipitates efficiently. Mn, Cr, and Zr form compounds with Mg and/or Si. In this connection, the inventors of the present invention have found that Mn, Cr, and Zr form compounds, in particular, with Si in larger amounts; that Mn, Cr, and Zr, if present in excessively high contents in a relationship with the Si content, cause Si-containing precipitates to be formed in a smaller amount during artificial aging, and this causes the aluminum alloy sheet to have lower bake hardenability.

Mn, Cr, and Zr form compounds with Si to reduce the amount of solute Si. For better bake hardenability also in consideration of this, not only a simple balance in content between Mg and Si, but also a total balance among the contents of Mn, Cr, and Zr in addition to the contents of Mg and Si should be controlled.

Specifically, if the member of the balance formula: [Mg]/[Si]-0.15[Mn]-0.3[Cr]-0.1[Zr] is less than 0.6, Mg atoms are inferior to form aged precipitates contributing to strengthening, and this causes precipitates to form in smaller amounts during artificial aging and causes smaller magnitude of bake hardening (lower bake hardenability), even when the contents of the individual elements fall within the above-mentioned ranges.

In contrast, if the member of the balance formula: [Mg]/[Si]-0.15[Mn]-0.3[Cr]-0.1[Zr] is greater than 1.8, Si atoms are inferior to form aged precipitates contributing to strengthening, and this causes precipitates to form in smaller amounts during artificial aging and causes smaller magnitude of bake hardening (lower bake hardenability), even when the contents of the individual elements fall within the above-mentioned ranges.

For these reasons, the contents of Mg, Si, Mn, Cr, and Zr are controlled to meet the condition represented by the balance formula: 0.6≤([Mg]/[Si]-0.15[Mn]-0.3[Cr]-0.1[Zr])≤1.8.

Other Elements

For still higher strength, the aluminum alloy sheet according to the present invention may further include at least one element selected from the group consisting of Cu in a content of 0.05% to 0.5%, Ag in a content of 0.01% to 0.2%, and Sn in a content of 0.005% to 0.3%.

These elements effectively allow the aluminum alloy sheet to offer better bake hardenability and to have higher strength and can be considered as equieffective elements for higher strength. Naturally, however, specific mechanisms of the effects of these elements are partially identical and partially different. Copper (Cu) and silver (Ag) are useful for offering better artificial aging hardenability (bake hardenability), promote precipitation of compound phases such as Guinier-Preston zone in grains in the sheet microstructure by artificial aging performed at a relatively low temperature for a relatively short time, and effectively allow the aluminum alloy sheet to have higher strength. Tin (Sn) captures atomic vacancies, thereby restrains Mg and Si from diffusing at room temperature, and restrains the aluminum alloy sheet from having higher strength at room temperature (natural aging at room temperature). Upon artificial aging, this element releases the captured vacancies, promotes the diffusion of Mg and Si, and effectively allows the aluminum alloy sheet to have better bake hardenability to thereby have higher strength.

However, these elements, if present in excessively high contents, typically form coarse compounds to impede sheet production and to cause the sheet to have strength, bendability, and corrosion resistance at lower levels. For these reasons, the contents of these elements, when to be contained, are controlled within the above-mentioned ranges.

Impurities

The other elements such as Fe, V, Ti, B, and Zn are unavoidable impurities, which tend to be incorporated typically from scrap as a raw material to be melted to form ingots, and are preferably minimized. However, these elements may be present in contents within the ranges allowable in standards such as Japanese Industrial Standards (JIS) in view typically of efficiency of melting and refining.

Grains

After controlling the aluminum alloy sheet to have the 6xxx-series alloy chemical composition as described above, the present invention specifies the microstructure of the 6xxx-series aluminum alloy sheet after T4 heat treatment. Specifically, the aluminum alloy sheet has a D1 of 100 μm or less and an aspect ratio D1/D2 of 3.0 to 10.0, where D1 is the average length of major axes of grains (average grain size in grain major axes) in an area of 1000 μm by 1000 μm as observed by SEM-EBSD in a cross section parallel to the sheet rolling direction; D2 is the average length of minor axes of the grains (average grain size in grain minor axes); and the aspect ratio D1/D2 is the ratio (aver-age axial ratio) of the major axes to the minor axes.

For good measurement reproducibility, the measurement of D and D2 is preferably performed in the following manner. The measurement in the cross section parallel to the sheet rolling direction is preferably performed at the sheet thickness central part as a measurement position. The measurement is preferably performed by SEM-EBSD technique at 75-fold magnification in a view field corresponding to the area (region) of 1000 μm by 1000 μm, where the SEM-EBSD technique refers to a technique using a field emission scanning electron microscope equipped with an electron backscatter diffraction pattern analysis system.

This condition on microstructure is an important, essential condition so as to allow the 6xxx-series aluminum alloy sheet to offer better bake hardenability and to have higher strength, on the preconditions that conventional aluminum alloy chemical compositions and production conditions are not significantly changed, and that formability is not lowered.

An aluminum alloy sheet, if having an average length of major axes of grains (average grain size in grain major axes) D1 greater than 100 μm and including coarse grains, fails to have better bake hardenability, even when the sheet has an aspect ratio within the above-specified range. The aluminum alloy sheet, if having an aspect ratio D I/D2 of less than 3.0, include grains as spherical, equiaxed grains as with conventional equivalents and fails to have better bake hardenability, where the aspect ratio D1/D2 is the ratio of the major axes to the minor axes.

In contrast, an aluminum alloy sheet, if having a high aspect ratio of greater than 10.0 due typically to excessively large D1 with respect to D2, fails to have better bake hardenability.

When D1 and D2 are measured by the SEM-EBSD technique, the grains on which D1 and D2 are measured or specified are grains with a tilt angle of 15° or greater. The grains with a tilt angle of 15° or greater are grains defined by large angle grain boundaries at a difference in crystal orientation (tilt angle) of from 15° to 1800, and the aspect ratio of such grains defined by large angle grain boundaries significantly affects the bake hardenability.

In contrast, of grains measured by the SEM-EBSD technique, grains defined by small angle grain boundaries with a difference in crystal orientation (tilt angle) of less than 15° less effectively contribute to better bake hardenability with a decreasing tilt angle, as compared with the grains defined by large angle grain boundaries.

Employment of grains defined by large angle grain boundaries with a tilt angle of 15° or greater as the object on which D1 and D2 are measured by the SEM-EBSD technique allows the aluminum alloy sheet to have better bake hardenability more surely.

Measurements of D1, D2, and Aspect Ratio

The D1, D2, aspect ratio, and average proportion of grains defined by large angle grain boundaries, as specified in the present invention, are all measured by the SEM-EBSD technique. The microstructure of the sheet in this case is measured at a position corresponding to the thickness central part (center part) in a cross section in the transverse direction (width direction) of the sheet, as with regular measurement position of microstructures of this type. For each factor, measured values of five measurement test specimens (at five measurement points) sampled at arbitrary positions in the thickness central part are averaged, and the average is defined and specified as the factor in the present invention.

The measurement areas (regions) are determined each as an area of 1000 μm by 1000 μm in a cross section parallel to the sheet rolling direction, at any three points in the thickness central part (center part). Electron beams are applied to the measurement area at a pitch of 1.0 μm, the lengths of major axis D1 and minor axis D2 of grains defined by large angle grain boundaries with a tilt angle of 150 or greater are measured, while assuming that the grains are spheroidal, and the measurements are averaged in the above manner.

The SEM-EBSD technique is a crystal orientation analysis technique, is widely used as a texture measurement technique, and employs a field emission scanning electron microscope (FESEM) equipped with an electron backscatter diffraction pattern (EBSD) analysis system. This measurement technique offers high measurement precision due to high resolution, as compared with other texture measurement techniques. Advantageously, this technique can measure the D1, D2, and aspect ratio at an identical measurement position of the sheet simultaneously and highly precisely.

According to the SEM/EBSD technique as disclosed, an aluminum alloy sheet sample is set in a lens barrel of the FESEM (FE-SEM), to which electron beams are applied so as to project an electron beam back scattered diffraction pattern (EBSD) onto a screen. A photograph of this is taken with a highly sensitive camera and captured as an image into a computer. In the computer, the image is analyzed and compared with patterns obtained by simulation on known crystal systems, and on the basis of the comparison, crystal orientations are determined. The determined crystal orientations are recorded as three-dimensional Eulerian angles typically with position coordinates (x, y, z). This process is automatically performed and repeated on all measurement points, and gives crystal orientation data at several tens of thousands to several hundreds of thousands of points upon the completion of measurement.

Advantageously, the SEM/EBSD technique has a wider observation view field as compared with electron diffractometry using a transmission electron microscope and can obtain information of the D1, D2, and aspect ratio on many grains in a number of several hundreds or more within several hours, as described above. In addition and advantageously, the SEM/EBSD technique performs the measurement not on grain-to-grain basis, but by scanning in a specified region at any constant interval, and can obtain the information at the many measurement points covering the entire measurement region. Such crystal orientation analysis techniques using an FESEM equipped with an EBSD analysis system are described in detail typically in Research and Development, Kobe Steel Engineering Reports, Vol. 52, No. 2 (September 2002), pp. 66-70.

The average grain sizes D1 and D2 respectively of major axes and minor axes are each calculated according to the expression:

Average grain size=(Σx)/n

where n represents the number of measured grains; and x represents the grain size of each grain.

Production Method

Next, a method for producing the aluminum alloy sheet according to the present invention will be illustrated below. The aluminum alloy sheet according to the present invention may be produced by a production method including production steps, which are, by themselves, common or known steps. In the method, aluminum alloy ingots having the 6xxx-series chemical composition are made by casting, subjected sequentially to homogenization (soaking), hot rolling, and cold rolling to have a predetermined thickness, and further subjected to heat treatments such as solution treatment and quenching, to give the aluminum alloy sheet.

Of the production steps, however, predetermined steps axe performed under conditions within preferred ranges as described below, so as to give the microstructure as specified by aspect ratio in the present invention. The predetermined steps, if performed under conditions out of the preferred ranges, may hardly give the microstructure specified by aspect ratio in the present invention.

Melting and Casting Cooling Rate

Initially, in the melting-casting step, a molten aluminum alloy melted and adjusted to have a chemical composition within the 6xxx-series chemical composition range is cast by a melting-casting technique. The melting-casting technique may be selected as appropriate from common melting-casting techniques such as continuous casting technique and semicontinuous casting technique (direct chill casting (DC casting) technique). In this step, the average cooling rate in casting in the temperature region from the liquidus temperature down to the solidus temperature is preferably set as high (as fast) as possible, of 30° C./min or more.

If the temperature (cooling rate) in the high-temperature region in casting is not controlled, the cooling rate in the high-temperature region is inevitably low. Cooling, if performed at such a low average cooling rate in the high-temperature region as above, may give larger amounts of coarse crystals during the temperature range in the high-temperature region, and this may highly possibly impede the control of the microstructure specified by aspect ratio in the present invention within the range specified in the present invention.

Homogenization (Soaking)

Next, the cast aluminum alloy ingots are subjected to homogenization before hot rolling. This homogenization (soaking) is important not only for a common purpose, i.e., for homogenization of the microstructure (to eliminate or minimize grain segregation in the ingot microstructure), but also for giving the microstructure specified by aspect ratio in the present invention. For this purpose, the homogenization is preferably performed as double soaking or two-stage soaking.

In the double soaking, the work after first soaking is once cooled down to a temperature of 200° C. or lower including room temperature, further reheated, and held at that temperature for a predetermined time, followed by start of hot rolling. In contrast, in the two-stage soaking, the work after first soaking (first-stage soaking) is cooled down to a temperature not being lower than 200° C., but being equal to or higher than 200° C., and held at that temperature, followed by hot rolling start as intact at that temperature or ater reheated to a higher temperature.

The first soaking in the double soaking, or the first-stage soaking in the two-stage soaking is performed under conditions selected as appropriate, at a temperature within the range from 500° C. to lower than the melting point for a holding time of 2 hours or longer.

However, in the first or first-stage soaking, heating from room temperature to the soaking temperature is performed as rapid heating at an average rate of temperature rise of 80° C./hr or more. The average rate of temperature rise is increased as above in order to form fine particles containing Mn, Cr, and/or Zr dispersed therein (hereinafter also referred to as “transition-element-dispersed particles”) to control D1 and the aspect ratio within the ranges specified in the present invention. The heating, if performed slowly at a low average rate of temperature rise less than 80° C./hr, may cause the transition-element-dispersed particles to coarsen and may fail to give the microstructure specified by D1 and aspect ratio.

In contrast, unlike the common procedure, cooling from the soaking temperature down to room temperature after the first soaking, or cooling from the soaking temperature down to the cooling stop temperature of 200° C. or higher after the first-stage soaking is performed at a low average cooling rate (slowly) as low as possible, of 40° C./hr or less, and preferably 30° C./hr or less. This configuration is intended to promote precipitation and growth of the fine transition-element-dispersed particles during cooling so as to control the sheet to include the microstructure specified by D1 and aspect ratio.

The second or second-stage soaking is performed under conditions selected as appropriate at a temperature within the range firm the hot rolling start temperature to 500° C. for a holding time of 2 hours or longer. In the second soaking, the ingots after the first soaking and subsequent cooling are reheated, and then cooled down to the hot rolling start temperature, or reheated up to the hot rolling start temperature and held at a temperature around that temperature. Alternatively in the second-stage soaking, the ingots after the first-stage soaking may be cooled down to the hot rolling start temperature and held at a temperature around that temperature. The second or second-stage soaking is performed at a higher temperature as compared with the first or first-stage soaking temperature.

The average rate of temperature rise in heating up to the second or second-stage soaking temperature, and the average cooling rate in cooling from the soaking temperature after soaking are respectively as with the average rate of temperature rise and the average cooling rate in the first or first-stage soaking as mentioned above.

Hot Rolling

The hot rolling of ingots after homogenization include a rough rolling step of ingots (slabs) and a finish rolling step performed according to the target thickness after rolling. The rough rolling step and the finish rolling step may be performed appropriately using a rolling mill such as a reverse mill or a tandem mill.

The hot rough rolling start temperature, which is the hot rolling start temperature, is from 350° C. to 400° C. Hot rough rolling, if started at a temperature lower than 350° C., may fail to hot-roll the work. In contrast, hot rough rolling, if started at a temperature higher than 400° C., may cause the transition-element-dispersed particles to precipitate coarsely, and may highly possibly impede the control of the sheet to include the microstructure specified by aspect ratio.

After the hot rough rolling as above, hot finish rolling is performed preferably to an end temperature in the range of 300° C. to 350° C. The hot finish rolling, if performed to an excessively low end temperature lower than 300° C., may cause deterioration in productivity due to a high rolling load. In contrast, assume that the hot finish rolling is performed to a higher end temperature so as to reduce the amount of a deformed microstructure remained in the microstructure and to allow the work to include a recrystallized microstructure. In this case, the hot finish rolling, if performed to an end temperature higher than 350° C. may cause the transition-element-dispersed particles to precipitate coarsely, and may highly possibly impede the control of the sheet to include the microstructure specified by aspect ratio.

Cold Rolling

In the cold rolling, the hot rolled sheet is rolled into a cold-rolled sheet (including one in the form of a coil) having a desired final thickness. To further refine grains, the cold rolling is desirably performed to a cold rolling reduction of 60% or more. Process annealing may be performed between cold rolling passes for the same purpose as with the heat treatment (rough annealing).

Solution Treatment and Quenching

After cold rolling, solution treatment and subsequent quenching down to room temperature are performed. The solution treatment and quenching may be performed using a common continuous heat treatment line. However, to sufficiently dissolve elements such as Mg and Si and to thereby surely offer precipitation strengthening at high level, it is preferred that the work is held at a solution treatment temperature of from 530° C. to the melting temperature for 10 seconds or longer, and cooled down at an average cooling rate of 20° C./second or more in the temperature range from the holding temperature down to 100° C.

Pre-Aging: Reheat Treatment

After the solution treatment and cooling down to room temperature as above, the cold-rolled sheet may be subjected to pre-aging (reheat treatment) within one hour. Holding of the sheet at room temperature between the completion of quenching down to room temperature and pre-aging start (heating start), if performed for an excessively long time, may cause the Si-rich Mg—Si clusters to form due to natural aging at room temperature and impedes the formation and increase of the Mg—Si clusters having good balance between Mg and Si. To eliminate or minimize this, the holding time at room temperature is preferably minimized, and the solution treatment and quenching, and the reheat treatment may be performed successively with approximately no time lag. The lower limit of the holding time is not particularly determined.

In the pre-aging, the work is held at 60° C. to 120° C. for a holding time of 10 hours to 40 hours. This allows the Mg—Si clusters having good balance between Mg and Si as specified in the present invention.

The pre-aging, if performed at a temperature lower than 60° C., or for a holding time shorter than 10 hours, tends to hardly restrain the Si-rich Mg—Si clusters from forming and to hardly increase the Mg—Si clusters having good balance between Mg and Si, and tends to cause the work to have low yield strength after baking finish, as with the case where no pre-aging is performed.

In contrast, the pre-aging, if performed at a temperature higher than 120° C., or for a holding time longer than 40 hours, causes the formation of excessively large amounts of precipitation nuclei, thereby causes the work to have excessively high strength upon bend forming before baking finish and to tend to have inferior bendability.

The present invention will be illustrated in further detail with reference to several experimental examples below. It should be noted, however, that the examples are by no means intended to limit the scope of the invention; that various changes and modifications can naturally be made therein without deviating from the spirit and scope of the invention as described herein; and that all such changes and modifications should be considered to be within the scope of the invention.

EXAMPLES

Next, the present invention will be illustrated with reference to several examples below. Initially, 6xxx-series aluminum alloy sheets having different microstructures as specified by aspect ratio in the present invention were individually produced by changing chemical compositions and/or production conditions. The As yield strength, bake hardenability (paint bake hardenability), and bendability were individually measured and evaluated on the sheets after holding at room temperature for 100 days after their production. The results of these measurements and evaluations are given in Tables 1 and 2.

Specifically, the aluminum alloy sheets were individually produced, in which 6xxx-series aluminum alloy sheets having the chemical compositions given in Table 1 were produced while changing conditions such as soaking conditions, hot rough rolling start temperature, and solution treatment temperature, as given in Table 2. A blank field in an element content in Table 1 indicates that the content of the element is equal to or lower than the detection limit.

Sheet Production Conditions

Specifically, the aluminum alloy sheets were produced under conditions as follows. Initially, aluminum alloy ingots having the chemical compositions given in Table 1 were each made by melting and DC casting. In this process, casting was performed at an average cooling rate in the temperature range from the liquidus temperature down to the solidus temperature of 50° C./min, in common to each sample. Next, the ingots according to each sample were subjected to first soaking at the average rate of temperature rise from room temperature up to the soaking temperature, at the soaking temperature, for the soaking time, and at the average cooling rate from the soaking temperature down to room temperature as given in Table 2, and subsequently subjected to second soaking. The second soaking was performed under conditions common to each sample, where each sample was heated form room temperature up to the soaking temperature at an average rate of temperature rise of 100° C./hr, held at a soaking temperature of 480° C. for a holding time of 6 hr, and then cooled from the soaking temperature down to the hot rolling start temperature given in Table 2 at an average cooling rate of 20° C./hr.

After hot rough rolling, the works were subjected to subsequent finish rolling, in common to each sample, to an end temperature in the range from 300° C. to 350° C. and thereby rolled into hot-rolled sheets having a thickness of 5.0 mm. The aluminum alloy sheets after hot rolling were subjected to cold rolling at a reduction ratio of 60% without process annealing between cold rolling passes, and thereby rolled into cold-rolled sheets having a thickness of 2.0 mm.

In addition, the cold-rolled sheets were, in common to each sample, subjected to successive (continuous) heat treatments (T4) while recoiling and coiling the sheets using continuous heat treatment equipment. Specifically, in the heat treatments, each sample was heated up to the solution treatment temperature given in Table 2 at an average heating rate of 10° C./second, and, after reaching the target solution treatment temperature, held at that temperature for 5 seconds, and then cooled down to loom temperature via water cooling at an average cooling rate of 100° C./second. Immediately after the cooling, the samples were, in common to each sample, subjected to pre-aging at 100° C. for 8 hours. After pre-aging, the samples were slowly cooled (naturally cooled).

After these heat treatments, the samples were left stand at room temperature for 100 days to give final product sheets, and test sample sheets (blanks) were cut out from the thickness center of the final product sheets. The D1, D2, and aspect ratio of grain microstructures of the test sample sheets were measured by the above-mentioned method, in which the measurement object was grains defined by large angle grain boundaries with a tilt angle of 150 or greater. In addition, the strength and bendability of the test sample sheets were measured and evaluated.

The results of these measurements and evaluations are given in Table 2.

Bake Hardenability

Of the test sample sheets, the 0.2% yield strength (As yield strength) and the 0.2% yield strength (after BH) were measured by tensile tests. The 0.2% yield strength (As yield strength) is the yield strength of a T4 sheet, namely, a sheet before forming (shaping) and bake hardening (BH). The 0.2% yield strength (after BH) is the yield strength of a sheet after 2% stretching and subsequent artificial aging.

The 2% stretching simulated bend forming as forming of the material sheet into a structural component. The artificial aging (BH) was performed at 18.5° C. for 20 minutes. Table 2 sequentially presents the As 0.2% yield strength, the 0.2% yield strength after BH, and an increase of the 0.2% yield strength after BH from the As 0.2% yield strength, as the bake hardenability.

The tensile tests were performed at room temperature on JIS Z 2201 No. 5 test specimens (25 mm by 50 mm GL by thickness) sampled from the test sample sheets. In the tests, the test specimens were pulled in a direction parallel to the sheet rolling direction. The tests were performed at a gauge length of 50 mm and at a constant tensile speed of 5 mm/min until the test specimens ruptured. These mechanical properties were measured on five test specimens (N=5), and averages of the five measurements were defined as, and evaluated as, the mechanical properties.

Bendability

The bendability was evaluated on the test sample sheets. In the tests, test specimens having a width of 30 mm and a length of 35 mm were prepared from the test sample sheets, while the sheet rolling direction was defined as the lengthwise direction of the test specimens. The test specimens were subjected to 900 V bending at a bend radius of 2.0 mm under a load of 2000 kgf, in accordance with JIS Z 2248.

The surface conditions, such as the presence or absence of orange peel surfaces, fine cracks, and large cracks, of the V bent portion were visually observed, and visually evaluated (rated) according to the criteria as follows. Samples rated at 6 or more were evaluated as accepted.

-   9: no crack and no orange peel surface; -   8: no crack, but slight orange peel surfaces; -   7: no crack, but orange peel surfaces; -   6: few fine cracks; -   5: fine cracks; -   4: overall fine cracks; -   3: large cracks; -   2: large cracks and near rupture; and -   1: rupture.

Examples 1 to 7 in Table 2 had chemical compositions within the range specified in the present invention, where the chemical compositions meet the condition represented by the balance formula: 0.6≤Mg/[Si-0.15Mn-0.3Cr-0.1Zr]≤1.8 as presented in Table 1, and were produced under conditions within the preferred ranges. These examples therefore had aspect ratios within the range specified in the present invention, as demonstrated in Table 2.

As a result, the examples had a 0.2% yield strength of 300 MPa or more after 2% stretching and subsequent BH treatment at 185° C. for 20 minutes even after natural aging at room temperature, as demonstrated in Table 2 and were found to have excellent bake hardenability. In addition, the examples offered excellent bendability.

Comparative Examples 1 to 4 in Table 2 employed Alloy No. 1 in Table 1, as with the examples. In contrast, however, these comparative examples were produced under conditions one or more of which were out of the preferred ranges specified in the present invention, where the conditions include soaking conditions hot rough rolling start temperature, and solution treatment temperature, as given in Table 2. As a result, Comparative Examples 1 to 4 had aspect ratios out of the range specified in the present invention and were inferior in either one or both of bake hardenability after natural aging at room temperature and bendability as compared with Example 1, which employed the alloy having the same chemical composition, as demonstrated in Table 2

Among them, Comparative Example 1 underwent first soaking performed at an excessively low average rate of temperature rise. This sample therefore had an excessively low aspect ratio out of the range specified in the present invention, had low bake hardenability, and, in addition, offered low bendability.

Comparative Example 2 underwent cooling at an excessively high average cooling rate after the first soaking. This sample therefore had an excessively low aspect ratio out of the range specified in the present invention, included grains as equiaxed grains, had low bake hardenability, and, in addition, offered low bendability.

Comparative Example 3 underwent solution treatment performed at an excessively low temperature. This sample therefore had a D1 larger than 100 μm to find to include coarsened grains, and had an excessively high aspect ratio out of the range specified in the present invention. The sample was therefore inferior in bake hardenability itself because of having a high As 0.2% yield strength, although having a higher 0.2% yield strength after BH. The sample also offered poor bendability.

Comparative Example 4 underwent only single soaking. This sample therefore had an excessively low aspect ratio out of the range specified in the present invention, included grains as equiaxed grains, had low bake hardenability, and had an excessively low 0.2% yield strength after BH.

Comparative Examples 5 to 10 in Table 2 produced under conditions in the preferred ranges, but employed Alloy Nos. 8 to 13 in Table 1, whose alloy chemical compositions are out of the range specified in the present invention.

These comparative examples had an aspect ratio out of the range specified in the present invention, or were inferior in either one or both of bake hardenability after natural aging at room temperature and bendability as compared with the examples, as demonstrated in Table 2.

Comparative Example 5 employed Alloy No. 8 in Table 1 and had an excessively low Mg content. This sample therefore was inferior in bake hardenability to the examples.

Comparative Example 6 employed Alloy No. 9 in Table 1 and had an excessively low Si content. This sample therefore had an excessively low aspect ratio out of the range specified in the present invention, included grains as equiaxed grains, had low bake hardenability, and had an excessively low 0.2% yield strength after BH.

Comparative Example 7 employed Alloy No. 10 in Table 1 and had an excessively high Si content. This sample therefore failed to be produced into a rolled sheet due to hot rolling clacking.

Comparative Example 8 employed Alloy No. 11 in Table 1 and had an excessively low total content of at least one of Mn, Cr, and Zr. This sample therefore had an excessively low aspect ratio out of the range specified in the present invention, included grains as equiaxed grains, had low bake hardenability, and had an excessively low 0.2% yield strength after BH.

Comparative Examples 9 and 10 respectively employed Alloy Nos. 12 and 13 in Table 1 and had an excessively high total content of at least one of Mn, Cr, and Zr. These samples had a D1 greater than 100 μm, included coarsened grains, and had an excessively high aspect ratio out of the range specified in the present invention. These samples therefore had low bake hardenability, and, in addition, offered low bendability.

Comparative Examples 11 and 12 in Table 2 respectively employed Alloy Nos. 14 and 15 in Table 1. These alloys contained elements in contents within the specified ranges, but whose chemical compositions did not meet the condition represented by the balance formula: 0.6≤([Mg]/[Si]-0.15[Mn]-0.3[Cr]-0.1[Zr])≤1.8.

Comparative Example 11 had the member of the balance formula lower than the lower limit, as seen from Alloy No. 14 in Table 1. This sample therefore had low bake hardenability and had an excessively low 0.2% yield strength after BH.

Comparative Example 12 had the member of the balance formula higher than the upper limit, as seen from Alloy No. 15 in Table 1. This sample also therefore had low bake hardenability and had an excessively low 0.2% yield strength after BH.

The results of these experimental examples demonstrate that the conditions on chemical compositions and aspect ratio specified in the present invention should be met so as to allow 6xxx-series aluminum alloy sheets to have a higher 0.2% yield strength of 300 MPa or more after BH at 185° C. for 20 minutes, even alter natural aging at room temperature, without deterioration in bendability.

TABLE 1 Aluminum alloy sheet chemical composition (in mass percent; remainder: Al) Mn + Alloy Cr + number Mg Si Fe Mn Cr Zr Zr Sn Cu Ag Mg/[Si—0.15Mn—0.3Cr—0.1Zr] 1 0.78 0.98 0.18 0.41 0.18 0.59 0.90 2 0.80 0.96 0.19 0.45 0.45 0.2 0.90 3 0.84 0.96 0.16 0.49 0.12 0.61 0.96 4 1.00 0.71 0.15 0.20 0.05 0.25 0.5 1.55 5 1.26 0.96 0.23 0.24 0.09 0.09 0.42 0.1 1.42 6 0.82 1.33 0.18 0.18 0.05 0.23 0.64 7 0.84 0.98 0.25 0.21 0.18 0.39 0.02 0.94 8 0.45 1.02 0.19 0.43 0.43 0.47 9 0.78 0.50 0.20 0.40 0.40 1.80 10 0.69 2.13 0.23 0.28 0.12 0.40 0.34 11 0.80 1.01 0.21 0.10 0.10 0.80 12 0.78 1.04 0.22 1.47 1.47 0.95 13 0.83 0.96 0.22 0.49 0.48 0.15 1.12 1.15 14 0.61 1.18 0.15 0.33 0.10 0.43 0.55 15 1.14 0.65 0.21 0.41 0.11 0.52 1.97

TABLE 2 Aluminum alloy sheet production method First soaking conditions Average Hot rolling Solution Alloy Average rate of Soaking Soaking cooling start treatment number in Soaking temperature rise temperature time rate temperature temperature Category Number Table 1 pattern (° C./hr) (° C.) (hr) (° C./hr) (° C.) (° C.) Example 1 1 double 100 530 6 20 400 550 Comp. Ex. 1 1 double 60 540 4 20 390 550 Comp. Ex. 2 1 double 100 540 8 60 420 550 Comp. Ex. 3 1 double 100 520 6 20 400 510 Comp. Ex. 4 1 single 100 530 8 40 500 530 Example 2 2 double 150 520 8 20 400 550 Example 3 3 double 120 550 4 20 400 540 Example 4 4 double 100 530 6 20 400 560 Example 5 5 double 150 530 6 20 380 540 Example 6 6 double 80 510 6 40 400 560 Example 7 7 double 150 550 6 20 400 550 Comp. Ex. 5 8 double 80 520 6 20 400 550 Comp. Ex. 6 9 double 100 510 4 20 400 550 Comp. Ex. 7 10 double 120 550 6 40 400 — Comp. Ex. 8 11 double 100 550 6 40 400 550 Comp. Ex. 9 12 double 120 530 8 20 400 550 Comp. Ex. 10 13 double 150 530 6 40 400 550 Comp. Ex. 11 14 double 80 510 4 20 440 540 Comp. Ex. 12 15 double 100 520 6 40 380 540 Aluminum alloy sheet microstructure Aluminum alloy sheet properties Average Average Average Bake harden- grain grain grain As 0.2% 0.2% Yield ability as size in size in aspect yield strength increase in Bendability major axis minor axis ratio strength after BH yield strength in 90° V Category Number D1 (μm) D2 (μm) D1/D2 (MPa) (MPa) (MPa) bending Example 1 59.6 10.5 5.7 192 313 121 7 Comp. Ex. 1 40.6 15.8 26 226 318 92 5 Comp. Ex. 2 42.2 15.9 27 231 324 93 5 Comp. Ex. 3 134 10.6 126 266 331 65 3 Comp. Ex. 4 38.8 14.9 26 209 296 87 7 Example 2 47.2 11.4 4.1 200 325 125 7 Example 3 56.6 10.5 5.4 207 323 116 7 Example 4 66.2 10.7 62 214 332 118 7 Example 5 47.8 10.8 4.4 198 326 128 7 Example 6 58.6 10.5 5.6 188 301 113 6 Example 7 43.6 11.6 3.8 178 314 136 8 Comp. Ex. 5 48.9 11.8 4.1 158 276 118 8 Comp. Ex. 6 47.6 25.3 1.9 166 243 77 7 Comp. Ex. 7 hot rolling cracking Comp. Ex. 8 36.5 17.6 2.1 188 287 99 7 Comp. Ex. 9 162 11.1 14.6 177 248 71 3 Comp. Ex. 10 158 10.8 14.6 184 239 55 3 Comp. Ex. 11 50.8 12.5 4.1 148 242 94 8 Comp. Ex. 12 48.9 13.1 3.7 137 233 96 8

While the present invention has been particularly described with reference to specific embodiments thereof, it is obvious to those skilled in the art that various changes and modifications may be made without deviating from the spirit and scope of the present invention.

This application claims priority to Japanese Patent Application No. 2015-108595, filed May 28, 2015, the entire contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The present invention can provide 6xxx-series aluminum alloy sheets having higher strength, without deterioration in their bendability. As a result, the 6xxx-series aluminum alloy sheets can be applied to wider applications as other automobile structural components than panel components, where the other automobile structural components are exemplified typically by frame components such as frames and pillars; and reinforcements such as bumper reinforcements and door beams. 

1: A high-strength aluminum alloy sheet comprising, in mass percent: Mg in a content of 0.6% to 2.0%; Si in a content of 0.6% to 2.0%; at least one element selected from the group consisting of Mn, Cr, and Zr in a total content of 0.2% to 1.0%, Al and unavoidable impurities, wherein the contents of Mg, Si, Mn, Cr, and Zr satisfy a condition represented by: 0.6≤[Mg]/([Si]-0.15[Mn]-0.3[Cr]-0.1[Zr])≤1.8 wherein [Mg], [Si], [Mn], [Cr], and [Zr] are contents in mass percent respectively of Mg, Si, Mn, Cr, and Zr, wherein the aluminum alloy sheet has a D1 of 100 μm or less and an aspect ratio D1/D2 of 3.0 to 10.0, where D1 is an average length of major axes of grains in an area of 1000 μm by 1000 μm as observed by SEM-EBSD in a cross section parallel to the sheet rolling direction; and D2 is an average length of minor axes of the grains. 2: The high-strength aluminum alloy sheet according to claim 1, further comprising at least one element selected from the group consisting of: Cu in a content of 0.05% to 0.5%; Ag in a content of 0.01% to 0.2%; and Sn in a content of 0.005% to 0.3%. 3: The high-strength aluminum alloy sheet according to claim 1, comprising Mn. 4: The high-strength aluminum alloy sheet according to claim 1, comprising Cr. 5: The high-strength aluminum alloy sheet according to claim 1, comprising Zr. 6: The high-strength aluminum alloy sheet according to claim 1, comprising Mn and Cr. 7: The high-strength aluminum alloy sheet according to claim 1, comprising Mn and Zr. 8: The high-strength aluminum alloy sheet according to claim 1, comprising Cr and Zr. 9: The high-strength aluminum alloy sheet according to claim 1, comprising Mn, Cr and Zr. 10: The high-strength aluminum alloy sheet according to claim 2, comprising Cu. 11: The high-strength aluminum alloy sheet according to claim 2, comprising Ag. 12: The high-strength aluminum alloy sheet according to claim 2, comprising Sn. 13: The high-strength aluminum alloy sheet according to claim 2, comprising Cu and Ag. 14: The high-strength aluminum alloy sheet according to claim 2, comprising Cu and Sn. 15: The high-strength aluminum alloy sheet according to claim 2, comprising Ag and Sn. 16: The high-strength aluminum alloy sheet according to claim 2, comprising Cu, Ag and Sn. 